Oil storage tanks which include stationary storage tanks, tank cars, ocean going tankers, barges, pipelines or other structures for storing or transporting crude oil or petroleum products will over a period of time accumulate large amounts of sludge made up of chemical or hyrocarbon deposits which deposits occur particularly in crude oils or heavy oils. Such buildup of sludge over a period of time limits the load carrying or storage capacity of the vessel. Buildup of sludge in oil conveyors also constitutes excess dead weight when the conveyor is returned empty from a delivery point to a shipping point. This increases the cost of operation. In addition, the sludge can interfere with the discharging ports or discharging distribution lines positioned within the storage tank. For this reason, removal of the sludge becomes desirable in order to provide for sludge free exit ports from the storage tank itself. Consequently, the interiors of such storage tanks must be cleaned periodically to remove the accumulation of sludge.
Moreover, with the increasing cost of petroleum products, it has become necessary to utilize whatever hydrocarbon oils are available and to minimize waste. The amount of recoverable oil from sludge is high and therefore it is desirable that the sludge itself be removed from the storage tank and be reprocessed so as to permit the recovery of the usable oil contained therein.
Furthermore, vessels which carry or store crude oil or other petroleum products, are often utilized to carry other products besides oils or carry other grades of oils as well. Thus, it becomes necessary to cleanse the interior of such vessels prior to becoming loaded with other products or higher grades of oil. Vessels of this type must be cleansed to a gas free state from time to time so they may be entered for periodic inspection or for repair in the event of damage. Techniques for cleaning oil carrying vessels or storages tanks has involved the use of steam and water applied in large volumes through spray nozzles against the interior side walls of the vessels under high pressure. Occasionally chemicals are used in the cleaning process but generally such cleaning systems involve the use of large volumes of water as well. These techniques for cleaning have proved to be labor intensive, energy intensive and generally result in the formation of large volumes of oil containing waste water. Moreover, storage vessels cleaned utilizing these techniques are often not completely or fully free of hydrocarbon deposits and are not suitable for upgrading and carrying or storing other products without further hand cleaning of the interior of the vessel. For this reason, it is typically required that persons enter the storage tank facility tank itself and manually remove the remaining deposits on the storage tank floor or walls themselves.
The standard method for cleaning an oil storage tank requires the extensive use of manual labor inside of the tank. In tanks which are free of inside obstruction by any structural members or heating pipes, front end loaders can sometimes be utilized for the cleaning operation. However, this requires cutting of a large hole in the side of the tank to permit entry of the front end loader. More often, specially designed vacuum trucks are employed to remove the sludge from the manway opening. Laborers then using shovels, squeegees, and the like, must work inside the tank manually in order to remove the sludge to the pick up point.
As noted above, the principal method of cleaning settled sludge from oil storage tanks involves the following general approaches. The tank is drained to lowest possible level using on site drain pumps and pipes. Larger pumps and/or vacuum trucks are brought to the site to remove the remaining sludge. Thereafter steam is sparged into the sludge to heat, loosen and reduce the viscosity of the unmovable sludge. Water is next injected below the sludge to enhance flotation of the sludge itself. In instances where the tank is free from obstructions on the floor, an opening in the tank wall is made for a front loader tractor to enter and move the sludge out of the tank. Alternatively, spraying devices which can be employed consist of a centrally erected nozzle which uses a high pressure stream of circulating oil to dislodge the sludge as in the aforementioned methods noted above.
An example of a typical known process for the recovery of tanks sludge hydrocarbons is disclosed in a publication entitled "What does it take to squeeze profits from tank sludge" by the Baker Oil Recovery Corporation. This publication disloses the R.U.S.H. process which is intended to reduce exposure of plant personnel to dangerous elements by eliminating the need to enter the tank. After analyzing the particular oil tank facililty to be cleansed, a predetermined ratio of chemicals, water, solvent, and crude oil is determined which will be employed by the technicians to formulate a metastable oil-in-water emulsion so as to provide for ease and cleansing of the tank. Typically, normal manway covers are removed and replaced with specially-designed metal discs. Next, fluid injection lances are inserted through the metal discs and into the tank. Thereafter the predetermined volumes of chemicals, water and solvent are mixed and pumped through the lances, into the sludge, and then back into the blending tank. This circulation continues on a daily basis until the sludge is satisfactorily emulsified. Next the pumps are shut off and gravity is allowed to separate the oil and water phases. The water layer is then discharged for treatment by means of normal effluent systems while the recovered hydrocarbon is blended back into a fresh crude for reprocessing. However, despite the improvements by this recovery process, typically ten percent of the original volume of sludge remains in the tank bottom as a semi-solid residue. Thus, the Baker Oil Recovery System is not able to fully cleanse a tank but admittedly leaves behind a residue which typical injection methods are not able to cleanse.
Other known methods include the continual circulation of the oil contents of the tank by means of pressure pumps which are positioned within the tank. An example of such pumps is provided in the trade brochure entitled "The Butterworth P-43 Machine For Economical Sludge Control In Crude Oil Storage Tanks", copyright 1981, which involves a twin-nozzle cleaning machine designed for cleaning sludge from the bottom of crude oil storage tanks. Its operation involves directing submerged jets of oil horizontally across the bottom of the tank. In this fashion, the oil is constantly circulated so as to avoid the creation of sludge. Although this methoo constitutes a preventive example of operation in order to avoid the creation of sludge, the energy required to continually operate the pump eventually makes the method of operation undesirable. Moreover, there is no assurance that the bottom sludge formation is totally avoided.
Other typical cleaning devices and methods are disclosed in various patent literature. U.S. Pat. No. 3,556,407 is directed to a cleaning device for cleaning tanks of oil tankers. The device includes a flange which is provided on an upper deck UDF of an oil tanker. A jet nozzle body is mounted at the lower part of the device and includes two attached nozzles which are symmetrically separated by 180.degree. in the same plane. Cleaning water is introduced into the device from above the UDF and finally is emitted from the nozzles.
U.S. Pat. No. 3,628,670 is directed to a cleaning apparatus for oil tanks. The device is intended to prevent any sediment from building up on the bottom of the tank by the continuous and automatic sweeping action of an arm. The cleaning apparatus is positioned at the bottom of the tank and includes a bridge structure or frame having spider like members which are integrally attached to a hub. The ends of the members are fixably attached to the tank wall by welding or other suitable means. The frame serves to provide a hub for rotatably mounting the top end of a rod whose opposite end is rotatably mounted to the bottom of the oil tank. A concave arm is attached to the rod and has a ball roller at its outer end. In this fashion, the arm can be rotated along the bottom of the tank and thereby the settlement buildup is avoided or prevented. This device also renders unnecssary any scraping of the hardened sediment from the tank bottom.
Other typical oil cleaning tank devices are disclosed in U.S. Pat. Nos. 4,153,555; 4,117,564; 4,341,232.
We have invented an improved portable oil tank storage cleaning device which provides a safer and more efficient method of cleaning oil tanks and containers which periodically require, for reason of repair, change in stored oil, or loss of working volume, the removal of accumulated sludge.